The present invention relates to a brake system as well as to a method for controlling and/or monitoring a brake system pump.
German Patent 195 48 248 A1 describes a method and a device for controlling a pump of an electrohydraulic brake system. In the brake system, a hydraulic fluid is conveyed from an accumulator via a valve means to the individual wheel brake cylinders, the hydraulic fluid being conveyed to the accumulator by a pump. In order that the loading of the accumulator by the pump be as noiseless as possible, the pump can be driven at a mark-to-space ratio that can be stipulated in accordance with need.
German Published Patent Application No. 196 38 196 describes a system for monitoring a brake system having a controllable hydraulic pump that is located in a hydraulic circuit, and having at least one solenoid valve whose operating state can be altered in accordance with a control signal. In this context, by altering the operating state of the solenoid valve, the resistance to flow in the hydraulic circuit is influenced. Monitoring elements are provided which, when predetermined operating conditions exist, actuate a display device to indicate a fault, as a function of a detected slowing of the hydraulic pump in varying operating states of the solenoid valve.
For safety reasons, the pressure supply, is monitored especially in electrohydraulic brake systems. For this purpose, the absolute system pressure is continually monitored with respect to threshold values, as is the pressure and the pressure change rate when the accumulator is loading. While the pressure is being regulated (for example, in the context of an anti-locking system or an anti-spin regulation system), the result is that the accumulator undergoes a volume drain, which is impossible or at best very difficult to measure. When there is a simultaneous reloading of the accumulator, it proves impossible in conventional systems to carry out a precise monitoring of the pump effectiveness. In conventional systems, conclusions about the operation or the effectiveness of the pump can only be formed on the basis of a pressure increase in the accumulator.
In order to avoid generating noise, a pump of this type is not driven at 100% during a loading operation, but is generally operated in a clocked manner. However, during travel, a noise generated by the clocked pump is noticeable and disturbing, in particular when rotational speeds change, whereas noise is significantly less noticeable at a constant rotational speed.
An object of the present invention is to create a brake system in which the pump operation can be reliably monitored in a simple manner. In addition, it is the goal to make available a brake system that produces noise at as low a level as possible.
According to the present invention, it is possible, in particular, to operate an electrohydraulic brake system such that the pump effectiveness, i.e., particularly the pump rotational speed and the pump performance, can be evaluated and monitored even during a simultaneous volume drain from the accumulator. In addition, by determining the pump rotational speed, which is made simple by the present invention, a phase-regulated driving of the pump is possible in order to minimize the disturbing noises by improving the pump clocking. During the operation of the pump, pressure pulsations are generated whose temporal curve mirrors the periodic opening of the discharge valve of the pump, i.e., in an electrohydraulic brake system, for example, the accumulator loading pump. In this context, the period duration of the pulsations or the corresponding measuring signal corresponds to the duration of one revolution of the pump. The maximum fluctuation level is a measure for the pump performance at a preselected elasticity on the pump outlet side and at a preselected temperature of the hydraulic fluid or of the pressure medium. In a typical brake system, e.g., an electrohydraulic one, the clocked pump operates, for example, at rotational speeds of roughly 1500-3000 rpm, which corresponds to a period duration of 20-40 ms.
According to one preferred embodiment of the brake system according to the present invention, the pressure sensor is arranged directly at the outlet of the pump. As a result of this arrangement, and despite the presence of elasticities which are caused, for example, by reservoirs provided in the brake system and/or by bore holes, pressure pulsations can be measured in a very precise and reliable manner.
According to one preferred embodiment of the method according to the present invention, a smoothing-out, as well as an offset compensation, is carried out on a measuring signal obtained as a result of detecting the pressure pulsations. At pump rotational speeds of 1500-3000 rpm, it is possible to smooth out the signal, for example, by reading in the pressure sensor signal sufficiently frequently, for example, every 2 ms. An offset compensation can be achieved as a result of the fact that this signal has subtracted from it a signal that is filtered over a long term, for example, the average value of the signal over the immediately preceding 40-80 ms. In this context, the time duration between two positive zero crossings is a measure for the period duration. The maximum value, or the amplitude of the signal obtained in this manner, depending on the temperature of the pressure medium, is a direct function of the pump performance. For example, by comparing the measured signal values with the stored table values, it can be evaluated as to whether the pump performance conforms with the specified values, and therefore whether the pump is functioning normally.
It has proven to be advantageous to operate the pump in a clocked manner and to drive it at a time point that can be stipulated, in accordance with a zero crossing and/or an extreme value of the smoothed-out or offset-compensated measuring signal. As result of this measure, it is possible to minimize structure-born sound generation from the point of view of noise intensity. The pump can be operated particularly quietly if it is driven in a phase-correct manner.
According to the present invention, it is possible, in a simple manner, to monitor the efficiency of the pump on the basis of the level of the detected pressure pulsations, or of the amplitude of the measuring signal that is generated from this source.